1. Field of the Invention
The present invention relates to a common electrode driving circuit for use in a display apparatus such as an active matrix liquid crystal display apparatus, and in particular to a common electrode driving circuit for use in a display apparatus provided with a common electrode opposed to a plurality of pixel electrodes or a common electrode opposed to all of a plurality of pixels provided in the display apparatus.
2. Description of the Related Art
Briefly referring to FIG. 6, an active matrix liquid crystal display apparatus generally includes a first substrate 1, a second substrate 2 opposed to the first substrate 1 and a liquid crystal layer (not shown) as a display medium interposed between the first substrate 1 and the second substrate 2. The first substrate 1 has a plurality of pixel electrodes, data lines, gate lines and the other elements (not shown) disposed on a surface thereof opposed to the second substrate 2. The second substrate 2 has at least one common electrode (not shown) on a surface thereof opposed to the first substrate 1. The common electrode is connected to common electrode driving terminals 4a and 4b provided on the first substrate 1 through common electrode transfer resistances 3a and 3b provided between the first and the second substrates 1 and 2. The common electrode is connected to the first substrate 1 in order to drive the common electrode using the common electrode driving circuit provided on the first substrate 1.
Common electrode driving circuits are generally classified into two types: the one using a DC voltage and the one using an AC voltage.
FIG. 7 illustrates a conventional common electrode driving circuit 15 using a DC voltage. The common electrode driving circuit 15 is provided with a complementary circuit 10. The complementary circuit 10 supplies a current to a common electrode or absorbs a current from the common electrode so as to maintain the voltage V.sub.COM of the common electrode at a specified level, which is determined by the voltages V.sub.H and V.sub.L (V.sub.H &gt;V.sub.L).
FIG. 8 illustrates a conventional common electrode driving circuit 16 using an AC voltage. The common electrode driving circuit 16 is provided with an operational amplifier 11 and a complementary circuit 12. The operational amplifier 11 amplifies a P.sub.OL signal having a square waveform inputted to an inversion input terminal thereof and sends an amplified AC signal to the complementary circuit 12. The complementary circuit 12 supplies a current to a common electrode or absorb a current from the common electrode in response to the AC signal inputted thereto. In the common electrode driving circuit 16, a voltage applied to the common electrode is fedback to the inversion input terminal of the operational amplifier 11 in order to allow an output voltage from the complementary circuit 12 to have the same ideal square waveform and the same phase with those of the P.sub.OL signal inputted to the inversion input terminal. FIGS. 9(a) and 9(b) illustrate waveforms concerning an output voltage from a common electrode driving circuit using an AC voltage for use in an active matrix liquid crystal display apparatus. FIG. 9(a) illustrates a waveform of a horizontal synchronous signal; and FIG. 9(b) illustrates an ideal waveform of the output voltage (indicated by the solid line) for performing AC driving using a square wave which has a voltage V.sub.M (indicated by the one-dot chain line) as the central voltage level thereof.
The conventional common electrode driving circuits have the following problems:
In an active matrix liquid crystal display apparatus, a data signal is outputted to all data lines in every horizontal period to turn "ON" pixels connected to the gate lines which are ON at that time. Each of the pixels includes a pixel electrode and a common electrode opposed to the pixel electrode and thus acts as a capacitor. Due to such a structure, the resultant voltage V.sub.COM applied to the common electrode is influenced by the output voltage from the data driver to the data line, namely, the voltage applied to the pixel electrode, and thus is changed from a specified level. The degree of the voltage change of the common electrode depends on the level of the output voltage from the data driver. In a conventional common electrode driving circuit, the pixel electrode which is a part of the capacitor and the common electrode transfer resistance 3a (FIG. 6) are connected in series. Accordingly, even if a voltage having an ideal waveform is inputted to the common electrode driving terminal 4a, the waveform of the resultant voltage V.sub.COM applied to the common electrode is changed from the ideal waveform.
FIGS. 10(a) to 10(c) illustrate signal waveforms obtained by the conventional common electrode driving circuit 15 shown in FIG. 7. The solid line indicates a resultant voltage V.sub.COM applied to the common electrode; the two-dot chain line indicates an output voltage from the data driver; and the one-dot chain line indicates the central voltage and corresponds to an ideal voltage applied to the common electrode. FIG. 10(a) illustrates a waveform of a horizontal synchronous signal; FIG. 10(b) illustrates the waveforms obtained when absolute values of output voltages from the data driver to all the data lines are maximum; and FIG. 10(c) illustrates the waveforms obtained when absolute values of output voltages from the data driver to all the data lines are minimum. As is apparent from FIGS. 10(b) and 10(c), when the absolute values of the output voltages from the data driver to all the data lines are maximum, the resultant voltage V.sub.COM applied to the common electrode is greatly influenced by the level of the output voltages from the data driver. By contrast, when the absolute values of the output voltages from the data driver to all the data lines are minimum, the resultant voltage V.sub.COM applied to the common electrode is influenced only slightly by the level of the output voltages from the data driver.
FIGS. 11(a) to 11(d) illustrate signal waveforms obtained by the conventional common electrode driving circuit 16 shown in FIG. 8. The solid line indicates a resultant voltage V.sub.COM applied to the common electrode; the two-dot chain line indicates an output voltage from the data driver; the one-dot chain line indicates the central voltage V.sub.M ; and the dashed line indicates an ideal waveform for the voltage V.sub.COM applied to the common electrode. FIG. 11(a) illustrates a waveform of a horizontal synchronous signal; FIG. 11(b) illustrates a square waveforms of the P.sub.OL signal as a reference signal; FIG. 11(c) illustrates the waveforms obtained when absolute values of output voltages from the data driver to all the data lines are maximum; and FIG. 11(d) illustrates the waveforms obtained when absolute values of output voltages from the data driver to all the data lines are minimum. As is apparent from FIGS. 11(c) and 11(d), when the absolute values of the output voltages from the data driver to all the data lines are maximum, the resultant voltage V.sub.COM applied to the common electrode is greatly influenced by the level of the output voltages from the data driver. By contrast, when the absolute values of the output voltages from the data driver to all the data lines are minimum, the resultant voltage V.sub.COM applied to the common electrode is influenced only slightly by the level of the output voltages from the data driver.
The voltage applied to the pixel (hereinafter, referred to as the "pixel voltage") corresponds to a difference between a voltage applied to the pixel electrode and a resultant voltage V.sub.COM applied to the common electrode. Accordingly, even if the voltage of the data signal outputted from the data driver is kept at a specified level, the pixel voltage changes in correspondence with the voltage V.sub.COM applied to the common electrode when the capacitor composed of the picture electrode and the common electrode is charged. In other words, even if the data signal maintains the same level of voltage, the image displayed by the liquid crystal display apparatus is changed in the tone. For example, assuming that there are 1620 data lines in all, when the output voltages to 810 data lines among the 1620 are maximum and output voltages to the remaining 810 are minimum, the resultant voltage V.sub.COM applied to the common electrode is more influenced by the output voltage from the data driver than when output voltages to all the 1620 data lines are minimum. As a result, the pixel voltage is lower in the former case than the latter case. If the difference in the tone of the displayed image caused by such a low pixel voltage is visually recognized, such a phenomenon is recognized as "shadowing".
FIG. 12 illustrates an example of shadowing. The liquid crystal display apparatus shown in FIG. 12 is in a so-called "normally white" mode. A black image is displayed when the pixel voltage is maximum, and a white image is displayed when the pixel voltage is minimum. Shadowing hardly occurs when the pixel voltage is in the vicinity of the maximum level or in the vicinity of the minimum level. This is due to the electrooptical characteristics of the liquid crystal display; namely, the liquid crystal display shows a small change in the transmittance in response to the pixel voltage when the pixel voltage is in the vicinity of the maximum or the minimum level. When the pixel voltage is in such a range as to display an image having halftones, the transmittance of the liquid crystal display greatly changes due to a small change in the voltage, thus easily causing shadowing.
In a display screen shown in FIG. 12, three areas A, B and C corresponding to different common electrodes from one another are arranged vertically. For example, when the display apparatus is operated so that the whole areas of A and C, and two end portions of area B will have tone a and the central portion of area B will have tone c, the two end portions of area B are influenced by the central portion of area B having tone c. As a result, the two end portions of area B obtain tone b which is between tones a and c instead of the tone a. In this manner, a desirable tone cannot be obtained.